Human telomeres are comprised of TTAGGG repeats and a multisubunit protein complex termed shelterin that is required for the replication and protection of chromosome ends. Tankyrase 1 is a poly(ADP-ribose) polymerase (PARP) that regulates shelterin structure and function at telomeres. Tankyrase 1 poly(ADP- ribosyl)ates (PARsylates) the shelterin DNA-binding subunit TRF1, which leads to TRF1 ubiquitination and degradation, and to telomere elongation by telomerase. In addition to its role in telomere length regulation, tankyrase 1 is required after DNA replication in the G2 phase of the cell cycle to resolve cohesion between sister telomeres. Moreover, tankyrase 1 depletion leads to cell cycle arrest (in mitosis or in G1) dependent on the cell type. Thus, tankyrase 1 plays a key role in the replication of telomeres, the processing of replicated sister telomeres and cell cycle progression. Tankyrase 1 has the added complexity that it has a closely related homolog, tankyrase 2 (with which it may be redundant), as well as multiple binding partners and subcellular localizations that vary with the cell cycle. In this proposal we will first, elucidate the function of tankyrase 1 at telomeres and in cell cycle progression in normal human cells and tumor cells. Second, we will elucidate the function of tankyrase 2, including redundancy with tankyrase 1 and non telomere functions of tankyrases in human and mouse cells. And third, we will elucidate the regulation and function of tankyrase 1 by characterizing its binding partners and phosphorylation across the cell cycle. Understanding tankyrase 1 (and 2) function will provide insights into the molecular basis of telomere function and in addition may also reveal underlying connections that coordinate telomere function and cell cycle progression. Since telomere integrity plays a key role in aging and cancer, insights gained from this work will be relevant to clinical strategies in aging and cancer.